The invention relates to a surgical instrument for tensioning a cable-like, a band-like or a cord-like tensioning element.
The tensioning element can for example be a cable formed of metal wires, a thread compound formed of plastic filaments or a linear mesh-work of yarns. A tensioning element of this kind will also be designated as a band for short.
Dwyer developed an operating technique for the surgical treatment of curvatures of the spine (scoliosis) in which a series of adjacent vertebrae are tensioned together by means of cables and screws within the body of a patient to form a stiffened part of the spine. A cable tensioning device is used in this operation. A device of this kind is known from GB-A-1 551 707. This cable tensioning device comprises a first lever-like part and a second part which is formed as a gripper element for the cable. These two parts are connected to one another via a main joint. The second part is a lever system in which levers are connected to one another via three joints. Two of these levers form a clamping gap of the gripper element.
An alternative surgical instrument for tensioning bands was developed which is assembled from fewer mutually movable individual parts in comparison with the above-named cable tensioning device and which can be characterized as a manually actuatable lever system with the following features:
The lever system consists of two actuation levers which are jointedly connected by a third lever, and springs which are arranged between the levers. In this a stable basic position of the system is given by the springs, which are under mechanical stress, and by movement limitations which act between the actuation levers and the third lever. A laying-in opening for the tensioning element is arranged at a tip of the first actuation lever, and a first jaw to a clamping gap for the tensioning element is arranged at a tip of the second actuation lever. A second jaw to this clamping gap is located at the third lever. In the basic position of the system the tensioning element can be inserted into the laying-in opening of the first actuation lever and into the clamping gap at the second actuation lever. Through a manual pressing together of the actuation lever, the second actuation lever makes a pivotal movement with respect to the third lever as a result of which the clamping gap is narrowed, so that the tensioning element is clamped firmly. Through a further pressing together the distance between the tips of the two actuation levers increases. The third lever is formed symmetrically as a bow with two side plates, with the actuation levers being pivotally arranged between the side plates at journaling bolts and with the journaling bolts being secured in bores of the two side plates. The clamping jaw of the third lever is located at the crown position of the bow. Together with the other clamping jaw and parts of the side plates, a laterally closed passage for the band is given. In this system consisting of three levers, the manufacture of the joint bearings proved to be complicated and expensive.
It is an object of the invention to create an instrument which can be used as a band tensioning device and which is formed as a three lever system in such a manner that the tension acting on the band during the tensioning can be determined. In addition a form of this instrument should be provided for which the instrument can be assembled without problem from the three levers.
The surgical instrument for tensioning a cable-like, a band-like or a cord-like tensioning element comprises a manually actuatable lever system. Two leversxe2x80x94the first and the second actuation leverxe2x80x94are jointedly connected by a third lever. In a basic position of the system the tensioning element can be laid into a laying-in opening of the first lever and into a clamping gap at the second and at the third lever. One of the actuation levers is formed in one section as a beam which is elastic in bending. The extent of a shape change of the beam can be measured. A relationship between this shape change and a tension in the tensioning element is known. The tension can be set as a result of this knowledge and of a measurement of the shape change, or a means serving for the same purpose is provided for determining a torque which acts at the corresponding actuation lever. The basic position is preferably a stable position, namely as a result of at least one spring which is arranged between the levers or as a result of another means which exerts corresponding forces.
The third lever is formed asymmetrically and has only one side plate. Journalling bolts for two lever joints are parts of the third lever. A single spring is secured at the third lever. A clamping jaw of the third lever is the base surface of a U-shaped passage. The spring has two limbs which in each case lie in contact at the inner flanks of the actuation lever under a mechanical stress. One clamping jaw of the second lever dips into the U-shaped passage. A movement limitation which acts between the second and the third lever is given by a separately insertable abutment pin.